Recirculation of exhaust gases has been developed as a method for inhibiting formation and emission of oxides of nitrogen during the combustion process in an automotive engine. In general, it is desired to recirculate exhaust gases at a rate proportional to the rate of induction air flow. To accomplish that purpose, exhaust gas recirculation (EGR) control assemblies have included a valve pintle positioned to maintain the control pressure in the EGR passage upstream of the pintle equal to a reference pressure. Recirculation of exhaust gases has thus been varied with exhaust backpressure, which in turn varies with induction air flow, to provide exhaust gas recirculation substantially proportional to induction air flow.
In such prior EGR control assemblies, the pintle was positioned in accordance with a subatmospheric operating pressure that was regulated by a transducer. The transducer employed an air bleed valve to regulate the operating pressure; the bleed valve was carried on a control diaphragm subjected on one side to the control pressure in the EGR passage and balanced by atmospheric pressure on the opposite side and by the bias of a spring or other force producing member; the combination of atmospheric pressure and the spring or other bias formed the reference pressure. With such an assembly, when the induction air flow (and thus the engine exhaust backpressure) decreased and the control pressure accordingly started to fall below the reference pressure, the air bleed was opened to increase the operating pressure and cause the pintle to reduce exhaust gas recirculation; when the induction air flow (and thus the engine exhaust backpressure) increased and the control pressure accordingly started to rise above the reference pressure, the air bleed was closed to reduce the operating pressure and cause the control valve pintle to increase exhaust gas recirculation. The transducer thus varied the operating pressure so the pintle was positioned to maintain the control pressure equal to the reference pressure and thereby provide EGR as a proportion of induction air flow.
In some applications it may be desirable not only to provide exhaust gas recirculation as a proportion of induction air flow, but also to vary the proportion from one set of engine operating conditions to another. For example, during heavy load operation it may be desired to recirculate exhaust gases in a relatively high proportion to induction air flow, while during light load operation it may be desired to recirculate exhaust gases in relatively low proportion to induction air flow. Some proposals for changing the proportion involved use of a third valve element to adjust the area of the EGR passage upstream of the control valve pintle--with all the complexities attendant upon use of a third valve element. Other proposals for changing the proportion involved changing the reference pressure to establish a new control pressure; however, when the control pressure is changed to a value which differs from atmospheric pressure, the proportion of exhaust gases recirculated is no longer exactly constant but instead varies slightly with induction air flow.